This invention relates to polyvinyl butyral sheet for light-transmitting, layered safety glazing panels and more particularly to such sheet formulated to improve performance when a heat-wave-reflective or electrically conductive coating is included in such panels.
Light transmitting safety glazings for window, windshield, sunroof, skylight, intrusion security, showcase, picture frame and like applications are well-known. They include one or more rigid transparent panels such as glass combined in a laminate with an impact-dissipating plastic sheet such as plasticized polyvinyl butyral (PVB). It is likewise known to control the strength of the bond between the plastic sheet and rigid panel at a desired level since if too high the plastic sheet undesirably ruptures on impact and if too low, splinters of the rigid panel can separate from the glazing, and if glass, can injure a person in the surrounding area.
It is further known to incorporate heat-wave-reflective multi-layer coatings (sometimes called "stacks") into such safety glazings to reflect infra-red radiation while transmitting significant visible light. The effect is to reduce temperature increase from solar radiation within an area delimited by one or more of such safety glazing panels. Heating a metal layer of such multi-layer coatings by electrical conductance reduces the time required for defrosting or defogging. Representative structures for vehicle windshields are disclosed in U.S. Pat. Nos. 4,799,745 and 4,782,216. In such safety glazings, the initially exposed, uncovered top layer of the multi-layer coating often contacts the plasticized PVB sheet. Such top coating layer is often photosensitive in that after extended exposure to light, the initially strong bond between the sheet and top coating layer deteriorates.
As disclosed in U.S. Pat. No. 5,061,568 and European Patent Application 263623 published Apr. 13, 1988, this adhesion durability problem has been addressed by proposing a special cap layer on the heat-wave-reflective coating to contact the plasticized PVB, which cap layer is chosen for its capability to preserve the bond under the influence of light. Though overcoming the adhesion problem, such cap layers are usually deposited by an additional process sputtering step which can be slow and therefore costly to the total value of the safety glazing panel.